Production of pulp



March 14, 1933- T. L. DUNBAR 1,901,706

PRODUCTION OF PULP Filed April 16 1952 MIME/VTOR Patented Mar. 14, 1933UNITED STATES PATENT om moms LEONIDAS nurses, or wArnR'rowN, NEW YORK,ASSIGNOR TO CHEMIPULP PROCESS, INCORPORATED, or warnnrown, NEW YORK, ACORPORATION on NEW YORK rnonuorroiv or PULP Application filed April 16,1932. Serial No. 605,696. 1

This invention relates to the production of pulp, andv in particular tothe production of a super-quality sulphite pulp for use in themanufacture of paper.

It is an object of my invention to provide a process capable ofproducing a sulphite pulp from which may be made sulphite paper ofunusually, high strength' A further object is to provide a process forthe production of sulphite pulp capable of giving a higher yield of pulpthan is obtainable by processes now in use.

7 Further, it is my object to produce a highcellulose pulp from whichmay be made a. paper similar to kraft in strength, but more pliable thankraft. a A further object is the provision of a flexible process capableof variation of produc- 1 ing pulp with varying characteristics as maybe desired.

. A still further object is the provision of a sulphite process by theuse-of which a savmg of acid is effected.

The higher strength of kraft pulp is due primarily to residual lignin,which is-not removed by the kraft process. While my improved processremoves'the greater part of this residual lignin and pentosans, in

doing so it leaves a-cellulose of unusually high strength, therebyimparting highly desirable characteristics to the finished product.

Briefly, in the practice of my improved process the digester is firstfilled with raw material and bisulphite liquor in accordance with thedisclosure of my United States Patents Nos. 1,646,084 and 1,699,234 and-Dunbar et al U. S. Patent No. 1,699,056. A working pressure andtemperature sufficient to release extractives from the raw material isthen built up and the digester is closed in for a soaking. periodsuitable to the wood conditions as hereinafter pointed out. Thebisulphite liquor is then drained from the digester and may be conductedto a the acid settling tank or to other parts of.

prepared the .acid system where partially .liquor is contacted with]iquors om the digester. The raw materialis then subjected to abisulphite cook in accordance lime acid EEl'SSUED tower. This isshown byway of example a only, as the applicability of my invention is notlimited to the milk of. lime system, but is equally applicable tothelimestone tower system. 2 is a line leading from the tower 1 to thesettling tank 3. From the settling tank, the line4 leads to the pump 5,which forces the liquor into the acid storage tank 7 through the line 6.Th'epump 8 forces the liquor from the storage tank 7 through the line. 9to the eductor 10, where it is contacted with relief fluids from. thedigesters 15. Although four digesters are shown, any number may be used.

The drop or condenser leg 30 leads from the eductor 10 through theexchange heater 23 -to the bottom of the accumulator 11.

12 is a pump for forcing the bisulphite liq- 16 is a valved branchleading from the 7 top of each digester for venting relief gases andfluids through the header 17 to the eductor 10, while 22 is a valvedside relief branch .for conducting liquors from the digester to J V theheader 17. The valved branch line 25 i leads from the top of thedigester to the header 27, which header discharges into the acid storagetank 7. 26 is a valved branch off from the-line 25, venting to theatmosphere. The operation of these vent lines has been fully describedin my copending application Ser. No. 446,046, now Patent 1,859,384 datedMay 24, 1932. Liquor may be drained from the digesters through thestrainer.18,-the line 20 having therein the valve 19, and the header 21leading back to the settling tank 3.

In .the practice of my improved process, .[I may proceedas follows:

The digester having been charged with chips, the valve in the branch 26is opened and the digester is filled with hot bisulphite liquor from theaccumulator 11 by means of the pump 12,- line 13 and valved branch 14,in accordance with the disclosure of my co-pending application, Ser. No.446,046. When the digester has been filled, the valve in the branch 26is closed and a pressure of approximately 50 pounds is applied to thecontents of the digester by running the pump 12 until this has beenaccomplished. The valve in the line 14 is then closed and the pump 12shut down, and the chips are subjected to a soaking or penetrationperiod under pressure, with all the valves of the digester lines closed.

The duration of this soaking period, as well as the temperature, willvary with the kind and quality of the raw material, and with its initialmoisture content, this soaking period being preferably continued untilthe extractives (by which I mean the so called Water-solubles) arereleased from the material and all the raw material is placed in aconstant state as regards moisture content. The bisulphite liquorpenetrates the chips to some extent, leaving a deposit of acid salts.These soften the encrusting material and the subsequent digestionthereof is expedited. In addition, this initial penetration maypartially insulate against too drastic a digestive action in thesucceeding stage of'the process.

At the termination of this soaking period, the bisulphite liquor isdrained out of the digester through the valved line 20 and is returnedthrough the header 21 to the acid settling tank, where it is mixed withpartially prepared liquor from the acid system. The liquor is thenpumped from the settling tank to the acid storage tank for reuse.Normally, the acid system is operated continuously and therefore thecontents of the settling tank are being continuously pumped to the acidstorage tank. The extractives contained in the bisulphite liquor drainedthe extractives, resinous matter and lignin back to the settling tankare neutralized by an excess of alkaline base in the raw liquor. Afterthe soaking period has been completed and the liquor has been drainedfrom the digester, the digester is again filled with hot bisulphiteliquor, steam is admitted to the digestert-hrough a line (not shown) andthe cooking is carried on in the usual manner until a large part of thelignin content of the wood is removed. This stage of'my improved processdiffers, however, from the acid cook as practiced in the presentsinglestage process. In the single-stage process,

are substantially dissolved out by acid. This produces, particularlytoward the end of the cook, chemical compounds which have adeteriorating effect on the fibres. In my im- 1,9o1,7oo 1 proved processthe formation of these com-. pounds is to a large extent prevented, dueto the fact that upon their appearance in quantities which would injurethe cellulose, the cook is stopped, all or a part of the bisulphiteliquor is drained from the digester,

and the material is then treated in a third stage with precipitated lime(or other suitable alkaline) water.

The length of time required to. carry out the second. stage of myprocess is a function of the strength of the liquor and of thetemperature employed. \Vhere it is necessary to make short cooks, themaximum temperature employed will be approximately 10 C. higher thanwill be the case if more time is allowable. If the bisulphite liquorruns relatively high in free S0 the time required as well as thetemperature can be reduced, although when operating with abnormally highfree acid content difiiculty may be experienced in recovering the acideconomically. I have obtained excellent results by operating with abisulphite liquor of approximately (3% free S0 and approximately 1.25%combined.

I have found that it is sometimes desirable, in order to produce specialgrades of pulp, to repeat the cook, or second stage of my process. Inthis case, a part only of this cook is accomplished with each separatecharge of bisulphite liquor and consequently there is a progressiveincrease in the maximum temperature for each recooking. For example, Ihave found that in the production of a sulphite pulp with the strengthcharacteristics of a high grade kraft pulp, the cook itself may becarried out in three stages, the maximum temperature during the firstbeing 110 (1., during the second approximately 120 C. and during thethird approximately 132 C.

After the completion of the cook, the valve 19 is again opened and allor part of the digester liquor is drained back to the settling tankfor'reuse, the solutes therein being neutralized by the addition of analkaline liquor. The digester is then filled up (as, for example, bymeans of the connection 32) with precipitated lime water, and thecontents of the digester are boiled or cooked under the heat stillremaining in the pulp until the desired pulp is obtained. It issometimes necessary to add more heat during the lime water cook, andthis may be accomplished in the usual manner by admisslon of steam intothe digester, or if the digester is provided with indirect heatingcoilsthe contents of the digester may be indirectly heated by theadmission of steam to these coils.

After this last cooking with precipitated lime water, the pulp may beblown into a blow pit or other suitable draining medium in the usualmanner, viz., normally the blowoff valve (not shown) at the bottom ofthe digester is opened and the contents of the digester are forced outby the pressure existing therein. In some cases it is possible to carryout the alkaline cook in the blow pit or elsewhere, after the stock hasleft the digester. The liquor that drains -ofl from the blow pit isconducted to settling or clarifying tanks for reuse.

The precipitated lime water has a mildly alkaline reaction andneutralizes any ligno sulphonic acids which may remain in contact withthe cellulose fibres after the acid cook. This action produces a muchstronger and more. pliable pulp than that produced by the single stageacid cook process and gives the pulpmany characteristics ofkraft pulpwhich is produced by the alkaline process. Q

The strength and folding characteristics of the pulp may be varied byvarying the point in the digestion process at which the precipitatedlime (or other alkaline) water is applied. For example, if a pulp isdesired having strength characteristics nearly the same as ordinarysulphite pulp, the second stage, or bisulphite cook, of my processismagnesium in addition to the calcium pres-' ent.

The applicability of my process is not limited to the spruce, hemlockand fir ordinarily used for the production of sulphite pulp. By myprocess I am able to produce a novel sulphite. pulp having an unusuallyhigh mullen, tear and fold test, but much more pliable than kraft pulp.

I claim 1. A process for-the production of pulp which comprises soakingfibrous material in heated bisulphite liquor under a pressure ofapproximately 50 pounds until the extractives are substantially removedtherefrom and the liquor has penetrated the chips, draining thebisulphite liquor from the fibrous material and mingling said liquorwith fresh liquor for reuse, digesting the fibrous material under heatand pressure in bisulphite liquor until the lignin is substantiallyremoved therefrom, withdrawing bisulphite liquor from the material andmingling it with fresh liquor for reuse, and cooking the fibrousmaterial in alkaline liquor until the incrustants of the cellulose aresubstantially removed.

2. A process for the production of pulp which comprises soaking fibrousmaterial in heated bisulphite liquor under'pressure until theextractires are substantially removed 1 I therefrom and the materlal isof uniform moisture content, withdrawing the bisulphite moved therefromand the liquor has penetrated the chips, draining off the bisulphiteliquor and mingling it with fresh liquor for reuse, digesting thefibrous material in bi-' sulphite liquor under, heat and pressure toremove lignin therefrom, draining off liquor from'- the material, andfinally cooking the material in precipitated lime water.

4. A process forthe production of sulphite pulp which comprises soakingfibrous material in bisulphite liquor, draining off the liquor,digesting said material under heat and pressure in additional bisulphiteliquor, and finally cooking said material in alkaline liquor.

5. process for the production of sulphite pulp which comprises firstsoaking wood chips in bisulphite liquor under pressure, then cooking thechips in bisulphite liquor under pressure at a temperature above 100 C.,and finally cooking in precipitated lime water.

6. In the production of sulphite pulp, charging a digester with fibrousmaterial, filling the charged digester with bisulphite liquorfrom anaccumulator and soaking the fibrous material in the digester under heatand pressure until the extractives are substantially put into solution,withdrawing liquor from the digester and returning it to theaccumulator, refilling the digester with bisulphite liquor from theaccumulator and digesting the fibrous material in the digester underheat and pressure to' remove lignin therefrom, withdrawing liquor fromthe i: gester, adding alkaline liquor to" the diadmitting precipitatedlime Water thereto and cooking the material therein.

8. In a process for the production of pulp, soaking fibrous material inbisulphite liquor 5 under heat and pressure until the extractives aresubstantially removed therefrom and the liquor has penetrated the chips,draining liquor from the fibrous material, then subjecting the fibrousmaterial to a series of digestion processes in bisulphite liquor underheat and pressure, andfinally cooking the fibrous material in alkalineliquor.

9. In a process for the production of pulp, soaking fibrous material inbisulphite llquor under heat and pressure until the extractives aresubstantially removed therefrom, then subjecting the fibrous material toa series of digestion processes under pressure and at successivelyhigher temperatures, and finally cooking the fibrous material inalkaline liquor.

10; In a process for the production of pulp, charging a digester withfibrous mate-- rial and soaking said material therein in bisulphiteliquor under heat and pressure to i remove extractives therefrom,Withdrawing liquor from the digester, refilling the digester withbisulphite liquor and digesting the fibrous material therein under heatand pressure to remove lignin therefrom, discharging the fibrousmaterial from the digest'er and thereafter cooking the fibrous materialin alkalineliquor. 4

11. A process for the production of sulphite pulp which comprises firstsoaking fibrous material in bisulphite liquor under heat and pressureuntil extractives are substantially removed therefrom and the materialis of uniform moisture content, then 40 digesting the fibrous materialin bisulphite liquor under heat and pressure until the formation ofligno sulphonic acid reaches a point deleterious to the material, andfinally cooking the fibrous material in alkaline liquor-until theincrustants of the cellulose are substantially removed therefrom.

12. Sulphite pulp obtained by soaking fibrous material in heatedbisulphite liquor under pressure until the extractives are substantiallyput into solution and the bisulphite has penetrated the fibrousmaterial, Withdrawing said liquor, cooking the material in freshbisulphite liquor under heat and pressure, Withdrawing said liquor, and

cooking the material in alkaline liquor.

" THOMAS LEONIDAS DUNBAR.

